1. Field of the Invention
The invention relates to radar signal reception and more particularly to clutter suppression for received radar signals.
2. Description of the Prior Art
The primary function of a weather radar system is to provide accurate estimates of the first three spectral moments: hydrometeor reflectivity, mean radial velocity, and doppler spectral width. It is well known that ground clutter seriously biases the radar estimates of these first three spectral moments of radar weather returns. Consequently, clutter suppression is essential when accurate spectral moment estimates are desired. Some clutter suppression techniques employed in the prior art tend to distort the radar signal spectrum, thereby biasing the weather radar signal spectral moment estimates and defeating the clutter suppression purpose.
Clutter reduction techniques exist for coherent and non-coherent radar systems. The clutter reduction techniques of the prior art for non-coherent radars provide less effective clutter suppression than do clutter reduction techniques of the prior art for coherent radars. Hence, the discussion to follow will be limited to coherent radar systems and clutter reduction techniques for suppressing clutter in such systems.
A frequently used coherent weather radar technique for Suppressing clutter passes coherently detected I and Q components of a temporal signal series through time-invariant high pass filters with clutter rejection bands around zero frequency. To preclude weather spectral distortion by the clutter filter, the lowest frequency in the weather spectra is positioned in the pass band of the filter at a frequency that is at least three times the 3 dB cut-off frequency of the filter. Clutter filters employed in the prior art systems are of two classes, finite impulse response (FIR) and infinite impulse response (IIR) time-invariant high pass filters. Time-invariant high pass filter design is straight forward. In these designs, steady state filter parameters are selected to achieve a specified stopband attenuation, stopband width, transition bandwidth, and passband ripple.
In weather radars, time-invariant high pass filters often perform more poorly than expected. Since time-invariant filters are designed on the basis of a received infinite pulse train, realized clutter attenuation for an actual finite pulse train, coupled to a weather radar receiver, may be less than the specified filter stopband attenuation. For example, it is not uncommon for an IIR filter designed for 50 dB stopband attenuation, operating on a finite length pulse train, to provide only 20 dB attenuation in the stopband.
A weather radar clutter suppression technique, of the prior art, which may provide greater clutter rejection than that provided by FIR and IIR filters, compares the shape and amplitude of the received signal autocorrelation function to the shape and amplitude of the autocorrelation functions of assumed models of weather and clutter to estimate the spectral moments of the received weather and clutter signals. This technique requires accurate models of clutter and weather signal returns. These models are difficult to achieve with sufficient accuracy to provide desired weather moment estimate accuracies.
A related technique compares the spectral shape of the received signal-plus-clutter to shapes of postulated signal-plus-clutter curves to extract the weather spectral moments. The accuracy of this technique depends on the accuracy of the assumed signal plus clutter models. Spectral modelling of signal plus clutter is no less difficult than that of autocorrelation modeling of weather plus clutter returns.
A clutter suppression technique which extracts an accurate representation of the clutter signal from signal plus noise is needed to provide accurate weather spectral moments from which accurate weather conditions can be determined.